Synonyms

Limb profiling; Occultation measurements

Definition

Limb. The portion of a planetary (or stellar) atmosphere at the outer boundary of the disk, viewed “edge on.”

Limb sounding. Atmospheric remote sounding technique involving observing radiation emitted or scattered from the limb.

Occultation. Atmospheric remote sounding technique involving observing radiation emitted (or reflected) by a distant body (solar, stellar, lunar, or an orbiting satellite), transmitted along a limb path through an absorbing and/or scattering planetary atmosphere, and detected by a remote observer.

Introduction

Limb sounding is a widely used atmospheric remote sounding technique, whereby the atmosphere is viewed “edge on” by a space- or airborne instrument. Limb sounding observations are made from the microwave and infrared – where thermal emission is observed – to the visible and ultraviolet, where observations are typically of sunlight scattered in the limb or of airglow. A wide range of spaceborne limb sounding instruments have been used to observe atmospheric temperature, composition, and dynamics from the upper troposphere (∼10 km altitude) to the mid-thermosphere (∼450 km). A closely related technique, known as “occultation,” involves observing the atmospheric absorption and/or scattering of radiation emitted by a remote source (solar, lunar, stellar, or, more recently a GPS satellite).

Limb sounding has significant advantages over nadir sounding (i.e., viewing straight down) or near-nadir sounding. Firstly, scanning the instrument field of view vertically across the atmospheric limb can give atmospheric profile information with greater vertical resolution than is typically possible from nadir sounders. In addition, complexities associated with emission or reflection of radiation by the planetary surface can be avoided. Finally, by viewing a significantly longer atmospheric path than nadir sounders, limb viewing instruments can achieve a stronger signal to noise for observations of tenuous atmospheric trace gases. However, this same long path length (typically a few 100 km) results in a poorer horizontal resolution than is possible with nadir sounding instruments.

With the exception of the infrared Mars Climate Sounder instrument (MCS, McCleese et al., 2007) on the Mars Climate Orbiter, limb sounding observations have been confined to those of Earth’s atmosphere and are the focus of the discussion in this entry.

Principles and techniques

Limb radiances and line broadening

Each limb view is associated with a particular “tangent height” – the closest distance from the limb ray to the Earth’s surface. High tangent height views typically give small signals, due to the tenuous atmosphere at these altitudes. As tangent altitudes decrease, atmospheric emission or scattering strengthens, increasing the observed signals. Eventually, the atmosphere becomes sufficiently opaque that signals from lower regions in the atmosphere are absorbed by the layers above and not seen by the instrument. At this point, radiances tend to remain fairly constant with decreasing tangent altitude (or to change only slightly, due to second-order geometrical effects) and are said to be “saturated” or “blacked out,” as the signal continues to derive largely from the lowermost nonopaque layers. Refraction is significant for limb rays in the lower atmosphere but is generally negligible above ∼20 km.

Molecular spectral lines are broadened in the atmosphere by a combination of the ensemble of Doppler shifts from the thermal motion of the molecules (“Doppler” broadening) and by collisions with other molecules (“collision” or “pressure” broadening). The latter generally dominates line widths of infrared and microwave signals up to ∼60 km, while for visible and ultraviolet wavelengths, Doppler broadening dominates throughout the bulk of the atmosphere. Pressure broadening of spectral lines can provide valuable information on the vertical distribution of trace gases (in addition to the information gained by vertically scanning the instrument field of view) with frequencies further from line centers conveying information on lower regions of the atmosphere, where lines are broad enough to contribute to the observed signals. For wavelengths where pressure broadening is insignificant, vertical distribution information can still be obtained by observing in multiple spectral regions having different atmospheric absorptions (and thus penetration depths).

Solar occultation and related observations

Observation of the atmospheric absorption of solar radiation (“direct sun” measurements) has a long heritage in atmospheric science (e.g., the observations of ozone pioneered in the 1920s by Dobson). Solar occultation is a natural extension of these ground-based techniques (and similar observations from balloon and aircraft vantage points). An instrument on a low Earth-orbiting spacecraft can perform an occultation observation during sunrise and sunset on each of ∼14 orbits per 24 h period. Typically, occultation instruments observe a narrow portion of the solar disk and track this as it rises or sets through the atmosphere. The strong solar signal provides excellent signal to noise and obviates the need to cool the instrument or its detectors. The observations of the sun above the atmosphere, before sunset or after sunrise, can be used to ensure a stable instrument calibration.

A succession of solar occultation instruments have provided a long record of atmospheric composition observations including the Stratospheric Aerosol and Gas Experiment (SAGE) I, II, and III series of instruments (McCormick et al., 1989), the Polar Ozone and Aerosol Measurement (POAM) instruments (Lucke et al., 1999), and the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) (Russell et al., 1993). Occultation observations were also made by the Atmospheric Trace Molecule Spectroscopy (ATMOS) instrument flown on the Space Shuttle ATLAS program (Gunson et al., 1996), and the Improved Limb Atmospheric Spectrometer instruments (ILAS I and II) on the Japanese Advanced Earth Observing Satellites (ADEOS I and II). The Scanning Imaging Absorption Spectrometer for Atmospheric CHartograpy instrument (SCIAMACHY, Bovensman et al., 1999) on the European Envisat performs solar occultation measurements in addition to limb and nadir imaging. Most recently, the Atmospheric Chemistry Experiment’s Fourier Transform Spectrometer and Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (ACE/FTS and ACE/MAESTRO) instruments have been continuing and augmenting this record (Bernath et al., 2005). Currently, ACE is the only operating mission employing solar occultation, though other concepts are in formulation including plans to fly a SAGE III instrument on the International Space Station.

While offering good vertical resolution and outstanding signal to noise and calibration stability, solar occultation instruments are fundamentally limited by orbital geometry to making only ∼30 observations per 24 h period. While some instruments augment this coverage with observations of lunar or stellar occultations, these, by definition, have poorer signal to noise than the solar occultation observations.

As described below, limb sounding observations of atmospheric emission or of scattered solar radiation offer comparable vertical resolution to occultation but have the advantage that observations can be made on a near-global basis daily.

Radio occultation

Observations of the atmospheric occultation of signals broadcast by GPS are a more recent development. In this technique, observations of refractive phase shift, as opposed to atmospheric absorption in different spectral regions, form the basis for the measurement. GPS occultation yields information on atmospheric refraction, and in turn temperature and/or water vapor profiles. More information on this technique is given elsewhere in this volume.

Microwave limb sounding

Atmospheric microwave emissions are generally associated with molecular rotational transitions, theoretically enabling observation of any atmospheric species with a significant dipole moment. Microwave limb sounding instruments have made observations of a wealth of species in the frequency range from ∼60 GHz (5 mm wavelength) to ∼2.5 THz (120 μm). Microwave signals are unaffected by aerosols and all but the thickest clouds, as the wavelengths used are longer than the typical particle sizes. This enables microwave observations of atmospheric composition in a limb sounding geometry in regions that are too cloudy for observations at other wavelengths.

To date, five spaceborne instruments employing limb sounding at microwave frequencies have flown: The Microwave Atmospheric Sounder (MAS) as part of the ATLAS payload on the Space Shuttle (Croskey et al., 1992), the Microwave Limb Sounder (MLS) instruments on the NASA UARS and Aura satellites (Barath et al., 1993; Waters et al., 2006), the Submillimeter Radiometer (SMR) on the Swedish Odin satellite (Murtagh et al., 2002), and, most recently, the Submillimeter-Wave Limb Emission Sounder (SMILES) on the International Space Station (Ozeki et al., 2001).

Microwave instruments can achieve arbitrarily fine frequency resolutions, enabling individual transition lines to be resolved in great detail. The observations of line shape enable simultaneous observation of both atmospheric pressure (largely affecting the width of a given transition line in the pressure-broadening regime) and species abundance (largely affecting the line strength). By combining inferred atmospheric pressure information with limb tangent altitude information, and assuming hydrostatic balance, atmospheric temperature profile information can be obtained.

The field-of-view width for a microwave instrument is determined by the antenna size and wavelength employed, with narrower fields of view achieved for larger antennae and/or shorter wavelengths and somewhat large antennae dictated for many observations. For example, the Aura MLS instrument’s 1.6 m antenna has a field of view that is ∼3.5 km wide at the limb (full width, half maximum) at 200 GHz from a 700 km orbit. The lower vertical range of microwave limb sounding instruments is limited by continuum absorption from oxygen, nitrogen, and water vapor, with ∼8 km altitude typically being the deepest penetration.

Infrared limb sounding

As with microwave limb sounding, infrared instruments observe thermal emission from the atmosphere, in this case mostly arising from molecular vibrational transitions. Again, collisional broadening enables determination of atmospheric pressure at the tangent point. Although not all infrared limb sounding instruments have the spectral resolution to resolve individual line shapes, pressure information can generally still be obtained from broader-band measurements.

Scattering and emission from clouds pose a more significant limitation to infrared limb sounders than microwave instruments, particularly in the tropics where clouds are prevalent in the upper troposphere. In clear-sky regions, infrared limb sounders can typically penetrate a few kilometers deeper than microwave sounders, but continuum absorption is, again, the ultimate limitation to this penetration. Infrared instruments can more easily achieve narrower fields of view than those in the microwave, and this can translate into a finer vertical resolution for the geophysical observations. However, the detectors typically need to be cooled (e.g., to ∼70 K) in order to achieve a scientifically useful signal to noise.

Infrared limb sounding instruments have a long history in atmospheric science, starting with the Limb Radiance Inversion Radiometer (LRIR) on Nimbus 6, followed by the Limb Infrared Monitor of the Stratosphere (LIMS, Gille and Russell, 1984) and Stratospheric and Mesospheric Sounder (SAMS, Drummond et al., 1980) instruments on Nimbus 7. UARS included two infrared limb sounding instruments – the Cryogenic Limb Array Etalon Spectrometer (CLAES, Roche et al., 1993) and the Improved Stratospheric and Mesospheric Sounder (ISAMS, Taylor et al., 1993). More recent infrared limb sounders include the Michelson Interferometer for Passive Atmospheric Sounding instrument (MIPAS, Fischer et al., 2008) on ESA’s Envisat spacecraft and the High-Resolution Dynamics Limb Sounder (HIRDLS, Gille et al., 2008) on NASA’s Aura satellite.

The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED) mission (Russell et al., 1999) is another recently launched infrared limb sounder mainly focusing on the chemistry and structure of the upper atmosphere.

Visible and ultraviolet limb sounding

Limb viewing instruments at visible and ultraviolet wavelengths generally observe sunlight scattered by the atmospheric limb. These include the Optical Spectrograph and Infrared Imaging System (OSIRIS, Llewellyn et al., 2003) instrument on Odin and the planned limb sensor for the Ozone Mapping and Profiling Suite (OMPS) on the NPOESS (National Polar-orbiting Operational Environmental Satellite System) Preparatory Project (NPP). The SCIAMACY instrument on Envisat also includes a limb scattering capability. As collisional broadening does not significantly affect the (mainly electronic or vibronic) molecular transitions at these wavelengths, tangent pressure cannot be deduced from the observations, and the height registration of the resulting geophysical products is more critically reliant on independent knowledge of spacecraft pointing than is the case for longer wavelength observations.

In addition to limb scattering sounders, past instruments have observed visible atmospheric airglow emissions in the upper atmosphere. These include the Wind Imaging Interferometer (WINDII, Shepherd et al., 1993) and the High-Resolution Doppler Imager (HRDI, Hays et al., 1993) instruments, both on UARS, which used these observations to deduce upper atmospheric dynamics.

Inversion approaches for limb sounding instruments

Although scanning the field of view of an instrument up and down the atmospheric limb enables high-resolution observations of vertical profiles, the observed signals are (as with nadir sounding) affected by emission, absorption, and scattering throughout the ray path. Disentangling the impact of each atmospheric layer on the observed signal and deducing vertical profiles of temperature and composition is a nontrivial task, commonly known as a “retrieval” or “inverse” calculation.

A variety of techniques have been employed for limb sounding retrievals. The so-called onion peeling approach uses observations at the top of the limb scan to characterize the uppermost atmospheric region. This is then accounted for when characterizing the next layer down, using lower-altitude limb views, and so on. A drawback of this technique is that the resulting profile depends strongly on the knowledge of the uppermost regions, where signal to noise is typically poor.

The most commonly adopted approach for limb sounding retrievals is the well-established “optimal estimation” method (Rodgers, 2000), which seeks the atmospheric state that matches all the observed signals simultaneously (taking into account potential noise on each signal). Although more computationally intensive than simpler approaches, this need not be a barrier with modern computing capabilities. Indeed, the most computationally demanding part of the calculation is typically the “forward model” (the computation that estimates the signal that would be observed by the instrument for a given atmospheric state), which is a central part of all but the simplest retrieval approaches, and upon which the accuracy of the resulting geophysical profiles ultimately depends.

Inhomogeneity along the limb line of sight can introduce biases in limb sounding retrievals, particularly in regions of strong atmospheric gradients. Some retrieval methods employ an iterative approach, whereby horizontal gradient information from a first pass is considered in a later retrieval step. In cases where the instrument line of sight is aligned with the spacecraft velocity vector, successive limb scans take multiple views through the same region of atmosphere, enabling a “tomographic” approach to the retrieval calculation to be taken (e.g., Livesey and Read (2000)).

Notable findings from limb sounding observations

The near-global daily coverage and good vertical resolution of limb sounding instruments has provided a wealth of information on atmospheric structure and composition from the upper troposphere through to the thermosphere. The early observations from LIMS and SAMS set the stage, with zonal-mean information on the abundance of key stratospheric and mesospheric trace gases. The three atmospheric composition limb sounders (CLAES, ISAMS, and MLS) on UARS, along with the HALOE solar occultation instrument, provided valuable insights into the dynamics and chemistry of Earth’s stratosphere, most notably processes associated with chemical stratospheric ozone loss and the transport of air into and throughout the stratosphere. UARS observations also provided valuable information on the impact of volcanic gases and the resulting aerosols on the stratosphere, following the dramatic June 1991 eruption of Mt. Pinatubo in the Philippines (4 months before the UARS launch).

In addition to stratospheric and mesospheric composition observations, UARS MLS provided unprecedented information on water vapor and ice clouds in the upper troposphere. The Aura MLS and HIRDLS instruments have enhanced this record providing the first daily global observations of upper tropospheric ozone, carbon monoxide (MLS only), and nitric acid. The upper troposphere is an important region of the atmosphere for climate, as it is where water vapor (the strongest greenhouse gas) and ozone have their largest radiative impact.

Outlook

At the time of writing, the only limb sounding instruments in operation are Aura MLS, Odin SMR and OSIRS, and Envisat MIPAS. The SMILES instrument experienced a critical failure after ∼6 months of operation, although plans are in formulation for a possible fix. While several new limb sounding instrument concepts are under formulation, to date none have been confirmed for launch. The most mature is the ESA Process Exploration through Measurements of Infrared and millimeter-wave Emitted Radiation (PREMIER) mission, which includes infrared and microwave limb sounding instruments observing the upper troposphere and lower stratosphere.

Conclusion

Limb sounding instruments provide a wealth of information on the composition, structure, and dynamics of Earth’s atmosphere, through observations of emitted or scattered radiation in an “edge on” viewing geometry. Limb sounding offers a valuable combination of good vertical resolution and near-global daily coverage, using wavelengths ranging from the microwave to the ultraviolet, and can provide observations from the upper troposphere to the middle thermosphere. Limb sounding observations have led to important discoveries concerning key dynamical and chemical processes in Earth’s stratosphere (including those processes associated with the “ozone hole”), and in the upper troposphere where water vapor and ozone have their strongest greenhouse forcing.

Cross-references

GPS, Occultation Systems

Stratospheric Ozone

Trace Gases, Stratosphere, and Mesosphere